BROOKLINE, Massachusetts (CNN) -- The success of in vitro fertilization has brought widespread hope for couples having difficulty conceiving.

It has resulted in tens of thousands of children born since the procedure began on a wide scale in the mid-1980s.

But it also has left physicians grappling with a new dilemma: what to do with thousands of human embryos languishing in clinics after the procedure yields a successful pregnancy.

Doctors at fertility clinics typically take several human embryos from each couple to help them conceive a child. So at clinics around the country, an estimated 25,000 human embryos -- never used, no longer needed -- remain in storage after a couple's successful conception. Each is much smaller than the head of a pin, but present enormous questions of conscience.

Each, after all, is a potential human life. The couples that donated them either conceived with another embryo or gave up trying, but no one wants to just throw away the embryos.

At Boston IVF Center alone, more than 4,000 embryos remain in storage. The clinic has not destroyed any of them so far. Some doctors say they don't have the authority to do so in any case.

"We all started that way. As little cells. Invisible to the naked eye," Dr. Michael Alper said. "If those are the instructions and the wishes of the couple, we need to work with the couple to exercise their wishes.

"It's couple's decision. It's not our decision."

For Tim and Nancy McCabe, one of the thousands of couples who now face that question, options include using another embryo to have another child with them for themselves. Or they may donate one to another couple, a so-called "pre-birth adoption."

The McCabes conceived their son Liam, now 15 months old, through the procedure. They still have three embryos in storage.

"Everybody I've met, they all felt the same, the same way," Tim McCabe said. "Once you've had success, and then knowing you have those eggs there, I don't think anybody could just say, sure, dispose of them."

But some couples can't be found to make that decision -- a fact likely to make the dilemma more complicated, and more common, as more embryos remain in cold storage. And no one has yet made the tough call out on the mysterious frontier where our lives all began.